Previous studies of carbohydrate fermentation in the premature infant have shown: (1) High breath H2 concentrations indicative of lactose fermentation in the colon; (2) A low rate of fecal excretion of carbohydrate-derived carbon implying colonic salvage of fermentation products such as lactate and short-chain fatty acids (SCFA) (e.g., acetate). These data have relevance to the following questions involving nutrition of the premature infant: (1) Energy balance and how it is affected by antibiotic therapy and fermentation pathways. (2) Inter-relationships of post- conceptional age, lactose intake and fermentation pathways (level of H2 production; relative production of acetate versus lactate). (3) The relationship of colonic SCFA production to energy metabolism. The experimental subjects for this study will be premature infants, (28-32 wk gestational age and 1-4 wk post-natal age) fed a premature infant formula (carbohydrate: 100% lactose). The Specific Aims of this proposal are the following: (1) Experiment 1: To compare two methods of assessing fecal carbohydrate energy (carbon) excretion based respectively on the partition of fecal energy losses; fecal excretion of excess 13C after a single dose of (D-1-13C)-lactose. (2) Experiment 2: In weekly studies beginning at 1 wk of age, to assess the inter- relationships of gestational and post-natal age, lactose intake, breath H2 excretion, 13C enrichment of fecal acetate and lactate, and the rate of fecal excretion of SCFA'S and of the 13C derived from lactose. (3) Experiment 3: To determine whether short-term (5-7 d) broad spectrum antibiotic therapy and parenteral nutrition affects the inter-relationships in Experiment 2. (4) Experiment 4: Using a repeated, enteral-dose, (1-13C)-acetate, turnover technique, to assess the rate of total body acetate production and oxidation in subjects participating in Experiment 2 (1-2 wk of age and again at 3-4 wk). Ten infants with existing arterial lines and 10 young adult volunteers receiving lactulose will serve as controls to assess the methodology. Our Secondary Specific Aims include: (1) To assess carbon dioxide production and the efficiency of energy utilization in relation to factors studied in Experiments 2 and 4, particularly the relative 13C enrichments of lactate and acetate (after 13C-lactose) and their rates of fecal excretion. (2) To assess the small intestinal absorption of lactose by monitoring the 13C enrichment of breath CO2.